This invention relates generally to high accuracy, low-friction reciprocating drive mechanisms, and more particularly to improvements in the constructions illustrated and described in U. S. Pat. No. 3,831,460 dated Aug. 27, 1974, entitled ANTI-BACKLASH NUT, and U.S. Pat. No. 3,977,269 dated Aug. 31, 1976, entitled ANTI-BACKLASH, SELF-ALIGNING NUT. The improvements of the present invention can be embodied in the construction disclosed and claimed in the copending application identified above.
U.S. Pat. No. 3,831,460 involves an anti-backlash nut construction incorporating a nut body which is split longitudinally along an axial plane, and which has a single pair of transverse slots adjacent the ends of the longitudinal slots, such that limited radially inward movement of the nut halves can occur under the biasing action of a garter (coil) spring seated in an external annular groove at the end of the nut body. The inner surfaces of the nut halves are threaded, and the threads of the nut closely follow the threads of the screw shaft to the end that there is no looseness between the two parts, resulting in virtually zero backlash.
The above arrangement has been found to operate exceptionally well; only after very prolonged use was there a tendency for wear to occur at the points of contact of the threads. Even though bronze nuts and steel screws were used, and with lubricant applied to the threads, eventually the oil film dissipated, resulting in somewhat increased drag forces, reduced efficiency, and possible sacrifices in tracking accuracy. Also, due to the friction between the nut and screw, heat could be generated, especially at higher operating speeds. If temperatures increased substantially, the lubricant could thin out, which might further aggravate any tendency for heat build-up.
In the patented constructions the nut parts were spring biased into relatively firm engagement with the threads of the screw, which could give rise to appreciable drag between the parts. For some applications, such drag might be considered objectionable. Even where such forces could be tolerated, they could contribute significantly to the wear experienced by the engaging surfaces, especially after a very extended operation.
If the threads of the screw were exposed, dirt or debris could become lodged in the grooves, especially if the device was operated in an adverse or unfavorable environment.
In the other prior arrangements of which I am aware, there were almost always experienced similar problems involving dirt and contamination, excessive heat or drag forces, and consequent wear.